Barium or strontium hexaferrites are well known for their magnetic properties. In particular they exhibit a high coercive field, which is appreciated for the applications requiring such a high coercive field.
For some time, efforts have been made to develop magnetic particles, compositions or products such as discs, cards and tapes for applications requiring a good keeping of the information and/or a good resistance to falsification or accidental erasing. This is the case for credit cards, identity cards and control access cards incorporating magnetic tracks. Hexaferrites have good prospects in such applications and allow a high recording density. The preparation of barium or strontium hexaferrite particles is well known. They can be obtained by coprecipitating a ferric salt and a barium or titanium salt in alkaline solution and calcining the coprecipitate obtained at high temperature.
C. D. Mee and J. C. Jeschke in "Single-Domaine Properties in Hexagonal Ferrites", Journal of Applied Physics, Vol 34, No 4, 1271-2, 1963, disclose the preparation of such hexaferrite particles. This old process exhibits the disadvantage of giving coarse particles which are non-homogeneous, dispersible with difficulty and have a tendency to form agglomerates. These disadvantages render the particles inappropriate for the preparation of magnetic recording layers. Numerous processes have been recently proposed to remedy these disadvantages, such as hydrothermal synthesis or improved processes of coprecipitation. Current synthesis processes for obtaining Ba and Sr hexaferrites result in particles in the form of hexagonal platelets.
It is desirable to improve both the dispersibility and the magnetic properties of hexaferrite particles. In known precipitation processes a barium or strontium salt is contacted with a carbonate to coprecipitate the barium or strontium carbonate and iron hydroxide. Because barium and strontium hydroxides are partially soluble, when they are used in the coprecipitate the desired stoechiometry of hexaferrite cannot be obtained unless large excesses of Ba or Sr are used. Therefore hydroxides have been avoided and replaced by insoluble carbonates, even at neutral pH. During the washing of the coprecipitate, there is no loss of barium and strontium and the amount of reactants involved is the stoechiometric amount. The presence of carbonate in the coprecipitate, however, renders the final product heterogeneous. Because Ba and Sr carbonate grains are bigger than iron hydroxide grains, the iron-Ba/Sr mixture is not uniform sized and during crystallization, heterogeneities appear.
Japanese patent application 62052133 discloses the preparation of Ba hexaferrite particles by coprecipitating a ferrous salt in the absence of carbonate ions. The comparative example below shows that the particles obtained by that process do not exhibit the advantageous characteristics of the particles obtained according to the present invention, and in particular magnetic properties such as magnetization and switching field distribution.
The switching field distribution (SFD) characterizes the width of the switching field distribution. From the hysteresis loop (magnetic flux M versus magnetizing field H), the half-bandwidth value .DELTA.H of the curve dM/dH derived from the loop can be calculated for obtaining SFD.
The lower the SFD, the more the magnetic moments of the particles switch for a field close to the coercive field. These flux transitions upon magnetic recording will be thus better defined, the thickness of the recorded layer will be lower and better defined giving a lower background noise and a higher signal/noise ratio, in particular for higher information densities. To solve the problem of the increase of signal/noise ratio, the SFD should be decreased. A low SFD indicates also a narrow distribution of the particle sizes.